The following properties are required for the zeolitic adsorbent solid:
Maximum possible adsorption capacity of the zeolitic agglomerate, i.e. highest possible zeolite content, since the zeolite constitutes the microporosity within which the adsorption takes place.
Maximum possible transfer within the zeolitic agglomerate, i.e. minimum time for passage of a hydrocarbon molecule from the exterior of the agglomerate to the centre of the zeolite crystals of the zeolitic adsorbent.
Highest possible crushing strength of the zeolitic agglomerate, and in all cases above 1.7 MPa as measured by the Shell test of fixed-bed crushing strength, modified to adapt to the granulometry of the agglomerates.
Documents of the prior art describing the chemical and microscopic characteristics of the zeolitic adsorbents used for separating para-xylene are particularly numerous, for example (U.S. Pat. Nos. 3,558,730; 3,663,638 (Neuzil); U.S. Pat. No. 3,960,774 (Rosback); U.S. Pat. No. 6,706,938 (Roeseler); U.S. Pat. No. 7,820,869 (Priegnitz); U.S. Pat. No. 7,812,208 (Cheng); U.S. Pat. Nos. 6,410,815; 6,884,918 (Plee); WO08/009845; WO09/081023 (Bouvier); US2011/105301 (Wang)).
The general teaching concerning the chemical characteristics of these adsorbent solids is that it is necessary to use a zeolite of faujasite structure (zeolite LSX, X or Y) exchanged with barium (to at least 90%, expressed in degree of exchange) or exchanged very predominantly with barium and to a minor extent with potassium (for example from 2 to 33%). For separating meta-xylene, U.S. Pat. No. 5,382,747 (Kulprathipanja) teaches the use of a sodium zeolite Y or of a sodium zeolite Y partially exchanged with lithium.
The general teaching concerning the microscopic characteristics of the adsorbent is that it is possible to use crystals of zeolite LSX with a size preferably below 2 microns (number-average diameter) or crystals of zeolite X with a size preferably below 1.6 microns (number-average diameter). The size of the crystals of conventional zeolite Y is of the order of 1 to 3 μm, and U.S. Pat. No. 7,728,187 (Kulprathipanja) recommends using nanocrystals of zeolite Y, with size between 50 and 700 nm, to improve the mass transfer of the meta-xylene.
An additional characteristic of these zeolites is that they must be hydrated so as to have an acceptable microporous transfer (U.S. Pat. No. 5,107,062 (Zinnen)).
Obtaining a microporous transfer that is as high as possible is, however, inconsistent with obtaining a maximum possible adsorption capacity, since the presence of water in the zeolitic structure will reduce the total capacity for adsorption of the xylenes as well as the capacity for selective adsorption of para-xylene (U.S. Pat. No. 7,820,869 (Priegnitz) or meta-xylene (U.S. Pat. No. 5,382,747 (Kulprathipanja)).
Consequently, it is sometimes necessary to adjust the hydration of the zeolite to achieve a compromise between acceptable microporous transfer and acceptable selective adsorption capacity. The degree of hydration of the adsorbent can be adjusted by varying the conditions of final activation of the adsorbent (i.e. temperature, duration etc.), which constitutes the last step in manufacture of the adsorbent.
The extent of hydration of the adsorbent can be evaluated approximately by measuring the loss on ignition, which can be carried out on the adsorbent before it is fed into the unit and brought in contact with the xylenes. Typically a zeolite LSX or a zeolite X must have a loss on ignition at 900° C. between 4 and 7.7% and a zeolite Y must have a loss on ignition at 900° C. between 0 and 3%.
The general teaching concerning the macroscopic characteristics of the adsorbent is that the content of active material can be increased by converting the binder to zeolite under the action of a basic alkaline solution, so that the finished product contains a reduced amount of non-zeolitic phase, which can be quantified by reference to an adsorbent composed solely of zeolite, in the form of powder, on the basis of measurements of adsorption or on the basis of intensities of XRD peaks. Moreover, this conversion of the binder to active material in the sense of adsorption allows the mechanical strength of the agglomerate to be maintained (WO08/009845 (Bouvier)), which is necessary for withstanding the mechanical stresses during its application in the units.
Another general teaching concerning the macroscopic characteristics of the adsorbent is that the agglomerates are generally of granulometry between 0.2 mm and 1.5 mm and more particularly between 0.25 and 1.2 mm (16-60 Standard US Mesh size—U.S. Pat. No. 7,820,869 (Priegnitz)) or between 0.35 and 0.8 mm (Example 1 US2011/105301 (Wang)). This granulometric range is obtained by sieving and/or cycloning at the end of the agglomerate forming step. The documents cited above provide indications for the “size range”, i.e. the granulometric range of the agglomerates, and not for the average diameter of these agglomerates. WO08/009845 and WO09/081023 (Bouvier) teach that the agglomerates generally have a number-average diameter ranging from 0.4 to 2 mm and in particular from 0.4 to 0.8 mm.
In the process for separating xylenes by simulated moving-bed adsorption, the zeolitic adsorbent solid is brought in contact with the liquid feed stream (the feed) composed of the mixture of xylenes, containing ortho-xylene, meta-xylene, para-xylene and ethylbenzene.
By using a zeolitic adsorbent based on zeolite of faujasite structure with an Si/Al ratio between 1 and 1.5 (zeolite LSX, X) exchanged with barium (to at least 90% expressed in degree of exchange) or exchanged very predominantly with barium and to a minor extent with potassium, the para-xylene is then adsorbed in the micropores of the zeolite preferentially relative to all of the other hydrocarbon compounds present in the feed stream. The phase adsorbed in the micropores of the zeolite is then enriched with para-xylene relative to the initial mixture constituting the feed stream. In contrast, the liquid phase is enriched with compounds such as ortho-xylene, meta-xylene and ethylbenzene in higher relative proportion than that characterizing the initial mixture constituting the feed stream.
By using a zeolitic adsorbent based on zeolite of faujasite structure with an Si/Al ratio in the range 1.5<Si/Al<6 (zeolite Y) exchanged with sodium or exchanged with sodium and lithium, the meta-xylene is then adsorbed in the micropores of the zeolite preferentially relative to all of the other hydrocarbon compounds present in the feed stream. The phase adsorbed in the micropores of the zeolite is then enriched with meta-xylene relative to the initial mixture constituting the feed stream. In contrast, the liquid phase is enriched with compounds such as ortho-xylene, para-xylene and ethylbenzene in higher relative proportion than that characterizing the initial mixture constituting the feed stream.
The liquid phase is withdrawn from contact with the adsorbent, thus forming a raffinate stream.
The adsorbed phase, enriched with para-xylene or meta-xylene depending on the zeolite used in the adsorbent, is desorbed under the action of a stream of desorbent, and withdrawn from contact with the adsorbent, then forming a stream of extract.
Maintaining the hydration of the zeolite at the desired value, for example a loss on ignition from 4% to 7.7% for zeolite X or LSX and 0 to 3% for zeolite Y, while it is used in the process for separating xylenes by simulated moving-bed adsorption, is assured by adding water to the incoming streams of the feed and/or of desorbent. The addition of water necessary for these levels of loss on ignition is such that the water content by weight in the hydrocarbon effluents (the stream of extract or of raffinate) is between 0 ppm and 150 ppm (40-150 ppm when the adsorbent is based on zeolite X or LSX and 0-80 ppm when the adsorbent is based on zeolite Y).
In the process for separating xylenes by simulated moving-bed adsorption, the zeolitic adsorbent solid is employed in one or two multistage columns for bringing in contact with the liquid stream. Multistage column is the name used for a column consisting of a multiplicity of plates arranged along an approximately vertical axis, each plate supporting a bed of granular solid, with the different successive beds being traversed in series by the fluid or fluids employed in the column. A device for distributing the fluids is arranged between two successive beds, providing feed to each bed of granular solid.
The bed of granular solid can be blocked by the distributing device, but most often there is an empty space between the distributing device and the surface of the bed of granular solid downstream in order to allow a slight sagging of the distributing device.
In general, the operation of a simulated moving-bed column can be described as follows:
A column comprises at least four zones, and optionally five or six, each of these zones consisting of a certain number of successive beds, and each zone being defined by its position between a feed point and a withdrawal point. Typically, an SCC unit for producing para-xylene or for producing meta-xylene is supplied with at least one feed F to be fractionated (feed of aromatic hydrocarbons consisting of isomers with 8 carbon atoms) and a desorbent D, sometimes called eluent (generally paradiethyl benzene or toluene), and at least one raffinate R is withdrawn from said unit, containing the products of the feed the least selectively adsorbed and the desorbent and an extract E containing the feed product adsorbed the most and the desorbent.
Other points for injection and withdrawal can be added so as to rinse the distributing circuits, as described for example in U.S. Pat. No. 7,208,651. Adding these additional rinsing streams does not in any way change the operating principle of the SCC, and for the sake of brevity, we shall not add these additional points for injection and withdrawal in the description of the process according to the invention.
The points for feed and withdrawal are modified over time, displaced in the same direction from a value corresponding to one bed. The shifts of the different points of injection or of withdrawal can be either simultaneous, or non-simultaneous as taught in U.S. Pat. No. 6,136,198. The process according to this second operating mode is called VARICOL.
Conventionally, 4 different chromatographic zones are defined in a column operating in simulated counter-current (SCC).                Zone 1: zone of desorption of the feed product adsorbed the most, comprised between injection of the desorbent D and withdrawal of the extract E.        Zone 2: zone of desorption of the feed products the least selectively adsorbed, comprised between withdrawal of the extract E and injection of the feed to be fractionated F.        Zone 3: zone of adsorption of the feed product adsorbed the most, comprised between injection of the feed and withdrawal of the raffinate R.        Zone 4: zone situated between withdrawal of the raffinate R and injection of the desorbent D.        
To increase the productivity of the separation process, we shall try to obtain zeolitic adsorbents of the smallest possible diameter in order to increase the transfer.
The documents of the prior art describing processes for pharmaceutical separation (Gomes et al., 2006, Adsorption, Vol. 12, p. 375) report processes for chromatographic separation in the liquid phase using agglomerates from some tens of micrometers to 100 μm.
In these processes using agglomerates of very small size, the head loss ΔP is then high. In the processes for separating xylenes, such levels of head loss ΔP are uncommon. Nevertheless, it is noted that, surprisingly, the head loss ΔP is not a determining criterion. It will have an effect notably on the thickness of the walls of the adsorber and on the output of the installations.
In the process for separating xylenes by simulated moving-bed adsorption, the zeolitic adsorbent is supported by plates forming successive beds of granular solid traversed in series by the fluid or fluids employed in the column. A device is positioned between two successive beds for distributing the fluids for providing feed to each bed of granular solid, and most often there is an empty space between the distributing device and the surface of the bed of granular solid downstream.
The plates can be cut in panels of the parallel type (meridian panels) or radial type (pie-cut panels). Each plate is delimited by a lower grid and an upper grid. A space not containing adsorbent is situated between the lower grid of a plate and the top of the bed placed under the plate in question. This space is necessary to avoid any phenomenon of mechanical degradation of the adsorbent solid connected with sagging of the plate.
Inherently in distribution, there is a tangential component of flow which promotes the phenomenon of partial fluidization or of entrainment of the particles at the surface of the bed of granular solid, causing the formation of furrows or banks on the surface of the bed. This phenomenon is promoted by high velocities of circulation and small particle diameters.
The formation of banks has two harmful effects on operation of the unit. On the one hand, it perturbs the flow and generates delays of residence time, degrading the separation performance. On the other hand, it promotes crushing of the sieve by the plates or the plate supports. Now, the service life of the sieve is largely linked to the generation of fines through crushing of the sieve.
It was discovered that by choosing a particular size range of the beads for an agglomerated zeolitic adsorbent, it was possible to overcome such phenomena, and thus increase the service life of the sieve.
The present invention describes a process employing an agglomerated adsorbent whose particular granulometry is selected so as to avoid degradation of the performance of the process in the long term. By choosing the particular granulometry it is possible to avoid the formation of banks on the surface of the beds of granular solid in the column or columns of the simulated moving bed.
The present invention thus proposes a process for separating xylenes using an agglomerated adsorbent of reduced granulometry, preferably based on faujasite zeolite with an Si/Al ratio between 1 and 6, for producing para-xylene or meta-xylene at high purity with a permanently improved productivity, while avoiding degradation of performance over time.